It is well known in the valve art to supply valves for the introduction of a sample volume of material for analysis testing or other purposes. One type of composition analysis system which uses sampling valves is chromatography, wherein a liquid or gas of unknown composition is introduced at one end of a packed column containing a diffusing medium. For a multiple component gas or liquid stream, each component will diffuse through the medium at a different rate, emerging at the other end of the column at different elapsed times from introduction.
The composition of an unknown gas stream is typically measured by first purging the column with a pure carrier gas, introducing a measured sample of the subject gas stream at the entrance end of packed column, and measuring the elapsed time and volume of each emergence of noncarrier gas components from the exit end of the packed column. Based upon past experience with various types of diffusing media, one skilled in the chromatography art may accurately determine both the identity and mole fraction of the components of the subject gas stream.
It should be apparent from the foregoing discussion that the accuracy of the final component measurement is dependent upon the introduction of a precise amount of the subject gas stream at the appropriate time. Sample valves developed for this use have been adapted to establish not only the flow of carrier gas into the diffusing medium, but to also maintain a precise volume of the subject gas or liquid ready for introduction to the packed column upon demand.
A typical sample valve for such an application is actuable between a purge mode and an injection mode. In the purge mode, the valve maintains a flow of carrier gas through the diffusing medium while simultaneously causing a flow of the subject gas or liquid to pass through a sample loop of preselected volume. Upon actuation of the valve into the inject mode, the flow stream of carrier gas is redirected through the sample loop pushing the temporarily trapped volume of subject gas into the inlet of the packed column and through the diffusing medium.
As will be appreciated by those skilled in the art, such a transfer requires redirection of a plurality of gas or liquid flow streams in a precise and timely fashion. Moreover, as the amount of subject gas injected must be precisely controlled, it is further apparent that any type of leakage or other sample loss will adversely affect the accuracy of the final gas readings. Leakage of carrier gas or liquid into the sample loop prior to actuation of the sample valve will likewise affect analysis accuracy.
The chromatograph sample valves of the prior art are typically of the sliding plate type, wherein a movable plate having a plurality of flow passages disposed therein is reciprocated over a corresponding surface or surfaces having a plurality of flow openings disposed therein. Depending upon the particular position of the sliding plate, various flow openings may be placed in fluid communication for transferring flowing material therebetween.
Two functions are central to the successful operation of a sliding plate valve for this type of service. First the sliding plate and corresponding surface must be maintained in a sealing relationship, typically accomplished by urging the plate and surface together by the use of springs or other urging means. It will be appreciated by those skilled in the art that such urging force must be uniformly distributed over the sliding plate, so as to prevent the possibility of uneven wear and/or leakage between the plate and surface. Second, it is necessary to maintain the flow passages and flow openings in substantially accurate alignment during each actuation of the sample valve. Any misalignment would clearly result in partially obstructed, totally obstructed, or misdirected fluid communication between the flow openings. It should also be noted that the parts in sliding contact, i.e., the plate and surface, will experience frictional wearing during extended periods of operation, requiring the replacement or repair of either one or both members.
Sliding plate valves known in the prior art typically use springs mounted on the valve body or other fixed urging means to press the sliding plate against the corresponding face surface in a sealing relationship. Such an arrangement has proved less than satisfactory, particularly when the sliding plate is reciprocated between positions, moving relative to the fixed urging springs and experiencing variations in force distribution. Such variations may lead to uneven plate wear and leakage.
Alignment of the sliding plates in valves of the prior art is also a potential problem. Typically, prior art valves confine this motion of the sliding plate between fixed or adjustable side rails which can be subject to misalignment or wear. For compact sample valves having numerous, closely spaced flow openings and passages, the margin of error due to misalignment of the corresponding flow openings and passages is very small, requiring frequent servicing to maintain valve performance.
it is also the nature of process gas chromatography that the chromatograph analyzer be placed adjacent the gas stream being analyzed, often in locations remote from the process operator, and in a hostile environment. It is therefore desirable to minimize the frequency of replacing or repairing worn parts in such an analyzer, as well as reducing the amount of time necessary to effect such repairs. Another drawback of the sample valve designs in the prior art is that in order to replace the sliding plate, typically the component wearing out most frequently, it is necessary to disassemble the valve and urging means in order to release and gain access to the sliding plate.
An improved sliding plate valve over those known in the present art would therefore include enhanced means for uniformly and constantly urging the sliding plate into sealing contact with the face surface, aligning the sliding plate flow passages and the face surface flow openings during the actuation of the valve, and be well adapted for simple, quick replacement of wearing parts.